This invention relates to a system for controlling the flow of air through a duct, either to allow the air to escape from the duct by way of exit means along the duct or to prevent it from escaping, depending on the way internal structure in the duct is connected to a source of air. In particular, the invention relates to a duct with an interior, longitudinal, flexible sheet, opposite longitudinal edges of which are joined to the duct at lines of attachment extending along the duct.
In factories and similar buildings, it is common to remove stale and contaminated air by exhaust fans. This tends to reduce the atmospheric pressure in such structures, but the buildings are not airtight, and air seeps in by way of the many openings and crevices in the walls, doors, etc., to replace the exhausted air. In cold weather, the air entering the building in this way is likely to be experienced by people in the building as drafts and cold areas, particularly near the floor. At the same time, the manufacturing and other activity in the building, as well as radiant heat absorbed by the roof and walls, adds to any heat produced by fuel-driven heaters in the building. The density of the air is inversely proportional to its temperature, so the hotter, lighter air, however it is heated, rises to the top part of the building, further aggravating the discomfort produced by the cold drafts.
It is known that comfort in the building can be improved by deliberately bringing in air and distributing it in a controlled manner to make up for the air blown out. One way this has been done is to force the make-up air into a duct suspended just under the roof and to allow that air to escape through exit holes spaced along the duct. Such ducts are as large as 60" in diameter and as long as 550', but they may be made both larger in diameter and longer, as well as smaller and shorter. The exit holes are typically arranged in straight rows about 60.degree. to 90.degree. apart as measured relative to the axis of the circular duct. In order to make use of the heat in the air just under the roof, it is common to suspend the duct so that the air exit holes face upward and outward, with their axes about 30.degree. to 45.degree. from the vertical. This allows the make-up air to entrain and mix with air in the space near the ducts, thereby heating the emerging air and carrying it outwardly and then down to the locations in which people are working.
In the hot weather, it is not desirable to mix the incoming air with heated air just under the roof. Thus, when those in charge of the heating and ventilation of the building believe that winter has ended, it is common for them to arrange to have the duct released from its suspension means, rotated 180.degree. on its axis, and re-attached with the air exit holes pointing downward and outward. This allows the make-up air to blow down toward workers without first being heated. Conversely, when those in charge believe summer has ended, they arrange to have the duct returned to its winter position. Unhooking the duct, rotating it, and connecting it up again is a time-consuming, and therefore expensive, operation, even though the duct is typically made of fabric, which collapses to the thickness of two sheets when no air is blowing through it, and therefore, there is a tendency not to make the changeover until well after the first days of the new season have passed, which means that there are likely to be several days of discomfort before, and sometimes even after, each changeover.